Gareth McGlynn struck with 15 minutes left to give Derry victory in Tuesday night's League of Ireland Premier Division game at the Brandywell.
Steven Gray sent over a low cross and McGlynn fired home at the back-post to secure the three points.
City's Clive Delaney had an early header cleared off the line before keeper Gary Rogers saved from McGlynn and another Delaney header.
Garret O'Connor sent a shot over the bar for St Pat's with two minutes left.
The Candystripes remain four points behind leaders Bohemians, who beat Galway United 2-0.
But Cork City's 2-1 defeat at Drogheda United leaves Derry just a point behind the Leesiders in third.
Bray Wanderers secured a 3-1 home win over Sligo Rovers in Tuesday night's other game.
Stephen Kenny's team is back in action on Friday night with a trip to take on Sligo.
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